Post install hook

This PR extends the possible configurations of the post-install-hook with 'Install.ps1'. For backward compatibility the 'Install.ps1' will still be executed always if not defined otherwise. This PR is related to the issue #10. 

Added features:
- '-DoNotPostInstall' to prevent hook execution
- '-PostInstallHook' to define a post install hook
- added additional properties to directory.xml-format to configurate suppressing execution of Install.ps1 and defining the post-install-hook and post-update-hook (and the explicit ModuleName found in the archive)

Changed functionality:
- Get-PsGetModuleInfo returned the directory.xml entry `title` as `ModuleName` which is inconsistent with all the other usages of `ModuleName` in PsGet. `ModuleName` is now the same like `id` if `ModuleName` is not explicitly defined (special case, see TestModules/directory.xml). For most of the entries inside directory.xml `id` and `title` are the same. Title only usage should be on the website directory. There should only be little or any impact for PsGet users causes by this change. 

PR contains tests for the new functionality and they are executed against posh v2 and v3.

When the PR is merged, I will change https://github.com/psget/psget/wiki/How-to-add-your-module-to-the-directory to match the new feature.